1. Field of the Invention
The present invention relates to a voltage rise suppression circuit and a panel television or especially a panel television to which the voltage rise suppression circuit is adapted.
2. Description of the Related Art
In a power supply circuit that supplies a predetermined supply voltage using a transformer, a secondary voltage to be placed on a voltage output line is fed back in order to control a primary voltage. This kind of circuit includes an overvoltage protection circuit or an undervoltage protection circuit for the purpose of protecting the circuit from an abnormal voltage.
Referring to FIG. 5 to FIG. 9, a conventionally employed overvoltage protection circuit and undervoltage protection circuit will be described below. FIG. 5 is a block diagram showing a conventional overvoltage detection circuit and undervoltage detection circuit. FIG. 6 is a timing chart showing relative timings observed when the overvoltage detection circuit shown in FIG. 5 has detected an abnormality. FIG. 7 is a flowchart describing a process which a microcomputer follows for controlling the timings shown in the timing chart of FIG. 6. FIG. 8 is a timing chart showing relative timings observed when the undervoltage detection circuit shown in FIG. 5 has detected an abnormality.
Referring to FIG. 5, an overvoltage detection circuit 1 generally includes: a short-circuiting Zener diode D3 having a cathode thereof connected onto an output line of a power supply circuit 4 and having an anode thereof grounded; dividing resistors R5 and R6 having one terminal thereof connected onto the output line of the power supply circuit 4 and having the other terminal thereof grounded; a diode D4 having an anode thereof connected to a junction point between the dividing resistors; and a microcomputer having an overvoltage detection terminal thereof 3a connected to the cathode of the diode D4.
In FIG. 5, an output voltage 1 has a fraction thereof developed by the dividing resistors R5 and R6. After the voltage at the junction point is thus dropped, the resultant voltage is applied to the anode of the diode D4. The diode D4 transfers the voltage at the junction point to the overvoltage detection terminal 3a of the microcomputer. The dividing resistors are adjusted so that, for example, when the power supply circuit 4 operates normally, the voltage at the junction point will be 0.6 V. The voltage of 0.6 V is also applied to the overvoltage detection terminal 3a. On the other hand, when the power supply circuit 4 operates abnormally, the output voltage 1 rises, and the voltage at the junction point rises accordingly. Consequently, the voltage applied to the overvoltage detection terminal 3a rises. Moreover, even when the voltage at the junction point gets lower than it is normally, the diode D4 rectifies the voltage for fear an abnormal voltage may be applied to the overvoltage detection terminal 3a. 
Referring to the flowchart of FIG. 7 and the timing chart of FIG. 6, a process to be followed when the microcomputer detects an overvoltage will be described below. The process is such that while the microcomputer monitors the output line, on which the output voltage 1 is placed, via the overvoltage detection terminal 3a, if the voltage applied to the overvoltage detection terminal 3a exceeds 1.4 V, the microcomputer senses the overvoltage and decides whether the overvoltage is attributable to an abnormality in the power supply circuit 4. The process is repeatedly executed while the power supply is on.
When the process is initiated, the microcomputer resets a counter to 0 at step S10 and proceeds to step S20. At step S20, the microcomputer obtains a voltage applied to the overvoltage voltage detection terminal 3a. At step S25, the microcomputer decides whether the voltage obtained at step S20 is equal to or higher than 1.4 V. Unless the voltage exceeds 1.4 V, the microcomputer considers that there is no problem and terminates the process. A conceivable cause of a temporarily rise in a voltage is noise or the like. In contrast, if a voltage equal to or higher than 1.4 V is detected, control is passed to step S30.
At step S30, the counter value is incremented. At step S35, the counter value is checked to see if it is equal to or larger than 4. If the counter value falls below 4, a condition is unmet. Consequently, control is passed to step S45. On the other hand, if the counter value is equal to or larger than 4, that is, if 200 ms has elapsed since a voltage equal to or higher than 1.4 V is detected at the overvoltage detection terminal 3a for the first time, the condition is met and control is passed to step S40. The microcomputer transmits an off-state signal to the power supply circuit 4 so as to stop the power supply circuit 4, and then terminates the process. An overvoltage is detected four consecutive times at intervals of 50 ms after it is detected for the first time. This is intended to prevent malfunction derived from noise or the like.
At step S45, a decision is made on whether 50 ms has elapsed since a voltage exceeding 1.4 V is detected at step S25. If 50 ms has not elapsed, a decision is made at step S45. After 50 ms has elapsed, the process beginning with step S20 is repeated.
Referring to FIG. 5, an undervoltage detection circuit 2 generally includes: dividing resistors R7 and R8 having one terminal thereof connected onto the output line of the power circuit 4 and having the other terminal thereof grounded; a diode D5 having a cathode thereof connected to the junction point between the dividing resistors; and a microcomputer having an undervoltage detection terminal 3b thereof connected to the anode of the diode D5.
In FIG. 5, the output voltage 1 has a fraction thereof developed by the dividing resistors R7 and R8. The voltage at the junction point is thus dropped and then applied to the cathode of the diode D5. The diode D5 transfers the voltage at the junction point to the undervoltage detection terminal 3b of the microcomputer. The dividing resistors are adjusted so that for example, when the power supply circuit 4 operates normally, the voltage at the junction point will be 3.3 V. The voltage of 3.3 V is also applied to the undervoltage detection terminal 3b. On the other hand, when the power supply circuit 4 operates abnormally, the output voltage 1 drops. The voltage at the junction point drops accordingly. This causes the voltage to be applied to the undervoltage detection terminal 3b to drop. Moreover, the diode D5 rectifies the voltage for fear an abnormal voltage may be applied to the undervoltage detection terminal 3b in case the voltage at the junction point gets higher than it normally is.
Referring to the flowchart of FIG. 9 and the timing chart of FIG. 8, a process to be followed when the microcomputer detects an undervoltage will be described below. The process is such that while the microcomputer monitors the output line, on which the output voltage 1 is placed, via the undervoltage detection terminal 3b, if the voltage applied to the undervoltage detection terminal 3b falls below 1.0 V, the microcomputer senses the undervoltage and decides whether the undervoltage is attributable to an abnormality in the power supply circuit 4. The process is repeatedly executed while the power supply is on.
When the process is initiated, the microcomputer resets the counter to 0 at step S50 and proceeds to step S60. At step S60, the microcomputer obtains a voltage applied to the undervoltage detection terminal 3b. At step S65, the microcomputer decides whether the voltage obtained at step S60 is equal to or lower than 1.0 V. Unless the voltage is lower than 1.0 V, the microcomputer considers that there is no problem and terminates undervoltage discrimination. A conceivable cause of a temporarily drop in a voltage is noise or the like. In contrast, if a voltage equal to or smaller than 1.0 V is detected again, control is passed to step S70.
At step S70, the counter value is incremented. At step S75, the counter value is checked to see if it is equal to or larger than 4. If the counter value falls below 4, a condition is unmet and control is passed to step S85. On the other hand, if the counter value is equal to or larger than 4, that is, if 200 ms has elapsed since the voltage equal to or lower than 1.0 V is detected at the undervoltage detection terminal 3b for the first time, the condition is met and control is passed to step S80. The microcomputer transmits an off-state signal to the power supply circuit 4 so as to stop the power supply circuit 4, and terminates the process. An undervoltage is detected four consecutive times at intervals of 50 ms after it is detected for the first time. This is intended to prevent malfunction derived from noise.
At step S85, a decision is made on whether 50 ms has elapsed since a voltage lower than 1.0 V is detected previously at step S65. If 50 ms has not elapsed, the decision is made again at step S85. After 50 ms has elapsed, the process beginning with step S60 is repeated.
Japanese Unexamined Patent Application Publication No. 11-127401 describes that: a secondary voltage or an output voltage of a switching power supply circuit is transferred to a microcomputer; the number of output lines of a counter circuit included in the microcomputer is two; a latching circuit is connected onto one of the output lines; and the output lines are led to an AND circuit. Herein, incorrect discrimination made by the counter circuit during a period during which the latching circuit holds a counter value is discarded. Thus, if the intermittent oscillation of the switching power supply circuit persists due to a current limiting circuit action, the stoppage of the switching power supply circuit derived from malfunction of an intermittent oscillation detection circuit is avoided.
In the foregoing conventional overvoltage detection circuit 1 and undervoltage detection circuit 2, as seen from the timing chart of FIG. 10, if an abrupt voltage rise occurs, an overcurrent flows into the circuit until the microcomputer detects the voltage rise. The Zener diode D3 or any other circuit element may be short-circuited and destroyed. If short-circuit destruction occurs, a voltage drops. The undervoltage detection circuit 2 detects the voltage drop, and turns off the power supply.
As far as a flap display panel (FDP) composed of multiple substrates is concerned, the substrates other than a power supply substrate may be broken. If many substrates are broken or whichever of the substrates is broken is unknown, it takes much time to complete inspection or repair. Moreover, if an FDP composed of substrates is purchased from a manufacturer for use, the FDP has to be entirely replaced with a new one.